object NonUnitForEach extends WartTraverser
This wart warns when methods scala.collection.IterableOnceOps.foreach and
scala.collection.IterableOnceOps.tapEach are used with non-unit valued functions.
This is needed because these methods' signature accepts any-valued functions.
So -Ywarn-value-discard does not warn about such discarded values.
scala.collection.IterableOnceOps.foreach is also used in for loops without yield
and therefore also covers cases such as for { x <- 1 to 10 } { x + 5 }.
Builder operations such as b += ... are identified by their return type this.type
and ignored when checking for non-unit return values. This allows common usages such
as for { x <- 1 to 10 } { b += x } and
for { x <- 1 to 10 } { if (x % 2 == 0) b += x }.
This does not work for all usages though; you can force a Unit type by
specifying the type argument of scala.collection.IterableOnceOps.foreach and
scala.collection.IterableOnceOps.tapEach as in (1 to 10).foreach[Unit] { ... },
or by ascribing Unit as in (1 to 10).foreach { x => f(x): Unit },
or by explicitly discarding the result: (1 to 10).foreach { x => f(x).discard }.
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